This invention relates to a process and apparatus for treating metal scrap to remove undesired inclusions prior to being melted, and deals more particularly with such process and apparatus specifically directed to the removal of both magnetic and non-magnetic tungsten carbide bits, and other magnetic and non-magnetic high density inclusions, from titanium machining scrap.
Titanium is a relatively expensive metal having a number of properties such as light weight, corrosion resistance, heat resistance, toughness and strength which make it the preferred structural material in many severe applications. For example, it is presently used widely in making rotating parts of aircraft engines, and other critical parts of aircraft, the failure of which could be disasterous. The material in question may be either commercially pure titanium or any one of a family of titanium based alloys, and as used herein and in the claims which follow the word "titanium" is intended to likewise refer to either such commercially pure titanium or any such titanium based alloy.
Titanium ingots from which parts are made are conventionally produced by electric arc furnaces using consumable titanium electrodes in inert atmospheres. The electrodes themselves may be made in various different ways and may contain both virgin and scrap titanium.
The titanium scrap used in electrode making is commonly divided into two different kinds. One of these kinds of scrap is referred to as "solid" scrap and consists of relatively large solid pieces of titanium such as the waste remaining after cutting desired pieces from titanium sheets, plates, rods, bars or the like. Since these scrap pieces are solid and relatively large it is easy to reliably separate from them pieces of foreign material which might contaminate the resulting melt. These solid scrap pieces are generally fastened to one another by welding or bundling to form electrodes or electrode portions.
The other kind of scrap used in electrode making is referred to herein as "machining" scrap, and is also sometimes broadly referred to as "turnings" scrap. It consists of the pieces produced by a machining operation, such as the chips produced by a milling or drilling operation or the turnings produced by a lathe cutting operation. Usually such pieces are small, but in the case of lathe turnings their size can vary widely and they are often of a curly or bushy form. In using machining scrap for electrodes it is briquetted, either by itself or in mixture with virgin or sponge titanium, and a number of briquettes are then welded or otherwise fastened together to complete each electrode.
Because of its properties, titanium is conventionally machined using tungsten carbide cutting tools and, due to the severity of the cutting procedure, it is not unusual for these cutting tools to fail or partially fail in service by having bits or pieces of their material break or flake away and fall into the scrap receptacle along with the scrap produced by the tool. These bits of tungsten carbide material which enter the scrap by breaking from cutting tools, and other similar high density pieces of foreign material such as stones which may possibly become inadvertently mixed with the scrap material, are highly undesirable in titanium scrap recycled into consumable electrodes. The reason for this is that when the electrodes are melted the tungsten carbide bits, and many other similar high density inclusions, unless of a very small size, do not melt at the temperature prevailing in the arc furnace and end up as weakening or failure-inducing anomalies in the parts made from the melt. Obviously, such weak points cannot be tolerated in rotating aircraft engine parts and other parts with severe service requirements and therefore scrap expected to evolve into such parts must be free of such non-melting inclusions.
Some tungsten carbide alloys used for cutting tools are fairly magnetic, and bits of such material can usually be reliably separated from titanium machining scrap with magnetic separators. However, other tungsten carbide alloys are non-magnetic or only slightly magnetic, and heretofore no process or apparatus has been known for reliably and efficiently removing bits of such low magnetic or non-magnetic tungsten carbide from machining scrap. Often it is impossible to assure that a given quantity of machining scrap contains no low magnetic or non-magnetic tungsten carbide inclusions, or other low magnetic or non-magnetic high density inclusions, and therefore the use of such machining scrap in making electrodes, at least those electrodes intended to be made into high quality parts, has often been prohibited and it has had to be used or sold as a downgraded material of considerably less monetary value than scrap known to be free of high density inclusions.
As used herein the term "free of tungsten carbide and similar high density inclusions" means free of such inclusions larger than a given size. Very small size pieces of tungsten carbide or similar high density materials, such as pieces smaller than 0.015 inch, melt or disintegrate in the arc furnace and cause no failure problems in the end products. Therefore, machining scrap can be taken to be free of high density inclusions even though it may contain particles of high density material smaller than 0.015 inch or smaller than some other specified small size.
The general object of this invention is, therefore, to provide a process and apparatus for efficiently treating titanium machining scrap to produce a scrap product reliably free of tungsten carbide bits and similar high density inclusions and suitable for use in titanium melts intended for making high quality or severe service parts such as the rotating parts of aircraft engines.
Another more specific object of the invention is to provide a cost-efficient process and apparatus, such as aforesaid, for treating titanium machining scrap to remove tungsten carbide bits and other high density inclusions and which process and apparatus are substantially more effective than any process or apparatus previously proposed for the same purpose, especially in its ability to remove non-magnetic or low magnetic inclusions as well as more highly magnetic inclusions.
Other objects and advantages of the invention will become apparent from the following description and associated drawing describing a preferred embodiment of the invention.
As described hereinafter, the process and apparatus of the invention include the use of an air separator of the type referred to by some as a "stoner" and by others as a "destoner". That is, the term "stoner" and the term "destoner" are used interchangeably in the art to refer to the same type of apparatus. For convenience, applicants have chosen to use the term "stoner" in the following description and in the claims, but it should be understood that for each usage of the term "stoner" appearing in the description and claims the term "destoner" may be used with equal effect.